An islanding mode refers to a state of a power grid in which a part of the grid is separated from the rest of the grid. The separated part of the grid may be powered by means different from those empowering the rest of the grid.
An ability to operate in island mode is useful in utilization of e.g. distributed power production. It enables utilization of electric power in areas where no public power transmission lines are accessible, but sources of electrical energy (e.g. small-scale hydro, wind, or solar power) are available. Island mode operation also enables power to be supplied to a part of the grid during a grid fault.
Known island converter control systems can control an island grid voltage through a direct control loop, for example, by acting on the converter flux or voltage. FIG. 1 illustrates an example of an island converter control system in accordance with a known implementation. A converter 11 is connected to a grid 12. The grid voltage ul is measured, and the magnitude |ul| of the voltage is determined. A controller 13 then uses the magnitude |ul|, together with a magnitude reference |ul*|, to calculate a value for a control reference. The control reference is used as an input for a control means 14. The control means 14 then controls an inverter bridge of the converter to produce a voltage vector on the basis of the control reference.
The controller 13 may, for instance, be a PI controller, as in FIG. 1. The control means may control the inverter bridge using, for instance, a PWM method or a DTC method. If a PWM-based approach is used, the controller 13 may produce a voltage vector magnitude reference. If a DTC-based approach is used, as in FIG. 1, a flux magnitude reference |ψc*| may be used to represent the control reference. In both approaches, the control means 14 may also use an angular frequency reference ωc* as an input.
A drawback of the control system presented in FIG. 1 is that a phase alignment angle of a voltage vector seen by the load cannot be controlled by the control system. At the same time, the load voltage vector is affected by the loading. Harmonic currents of non-linear loads may cause harmonic distortions to the output voltage. These currents may be difficult to compensate for. It may, however, be important to compensate for harmonic components of the output voltage of an island converter since a filter, and possibly a transformer, at the output of the island converter may cause the island grid to have a relatively large impedance. Under these conditions, it may be quite difficult to improve the quality of the voltages seen by the load.